During normal gait, the ankle joint, shank, and foot play important roles in all aspects of locomotion, including shock absorption, stance stability, energy conservation and propulsion. For example, FIG. 1 shows foot 10 and ankle joint 12 movement during part of normal gait. A gait cycle is typically defined from the initial contact of the heel 14 to the following heel contact. At the initiation of the gait cycle, impact forces are dissipated when energy is absorbed by the soft tissues at the heel 14 as the foot 10 comes into contact with the ground 16. Additionally, the muscles and tendons of the ankle joint complex act as an energy-dissipating brake to control the deceleration of the foot 10 before full contact with the ground 16 at foot-flat. The ankle joint complex also helps to maintain stability during stance phase. This is particularly important during the single support part of the stance phase, when the contralateral limb is swinging and only one limb is supporting the body. In addition to providing stability, energy is stored in the stretching of tendons and muscles of the ankle joint complex when the shank 18 pivots. The plantarflexion torque generated at the ankle 12 at push-off results in the highest power output for any joint during walking and is the primary source of power for forward propulsion.
Pathology or injury that affects the ankle joint can significantly impact quality of life by impairing some or all functional aspects of gait. Both dorsiflexor and plantarflexor muscle groups of the ankle-foot complex are critical to normal walking, and undesirable compensatory gait patterns result from weakened or impaired muscles of either type. Other causes of lower limb gait deficiencies include, but are not limited to, trauma, incomplete spinal cord injuries, stroke, multiple sclerosis, muscular dystrophies, and cerebral palsy.
The dorsiflexors (e.g., shin muscles) lie anterior to the ankle joint and include the tibialis anterior, extensor digitorum longus, and extensor hallucius longus. Weak dorsiflexors affect both stance and swing phases of gait, causing clearance issues during swing phase and uncontrolled deceleration of the foot at initial stance. Swing is affected because the foot does not effectively clear the ground due to weak or absent dorsiflexor muscles, which results in a steppage-type gait pattern that is commonly called foot drop. Steppage gait is a compensatory walking pattern characterized by increased knee and hip flexion during the swing phase so that the toe clears the ground during walking. The weak or absent dorsiflexors also prevent the controlled deceleration of the foot shortly after heel strike. This lack of control results in an often audible foot slap that impacts stance initialization.
The plantarflexors (e.g., calf muscles) lie posterior to the ankle joint and include the gastrocnemius, soleus, and the peroneal and posterior tibial muscles. From heel strike to middle stance, the ankle plantarflexors concentrically contract to stabilize the knee and ankle and restrict forward rotation of the tibia. At the end of stance, the plantarflexors concentrically contract and generate torque that accelerates the leg into swing and contributes to forward progression.
Weak plantarflexors primarily affect stance phase by reducing stability and propulsive power of the individual, particularly during limb support. Individuals with impaired ankle plantarflexors compensate by reducing walking speed and shortening contralateral step length. Reduced walking speed results in a corresponding reduction in torque needed for forward progression. The shortened contralateral step is thought to increase stability by limiting anterior movement of the center of pressure with respect to the ankle. Impaired individuals may maintain a fast walking pace by using their hip flexors to compensate for weak plantarflexor muscles.
Ankle foot orthoses (also referred to herein as orthoses or AFOs) can be used to ameliorate the impact to gait of impairments and injuries to the lower limb neuromuscular motor system. AFOs can be used for rehabilitation, diagnostic, or training devices, for example, to assist walking function, direct measurement of joint motion and force, and to perturb gait. Existing technologies for AFOs include passive devices with fixed and articulated joints with or without spring assist, semi-active devices that modulate the spring or damping about the joint, and active devices with various technologies to produce power and to move the joint.
Passive devices generally limit the foot angle to the neutral position (i.e., 90° between leg and foot), which can produce an unnatural gait but prevents further damage or injury and provides limited mobility to people that use them. Passive orthoses do not provide direct assistance during the propulsive phase of gait. Commercial passive devices improve gait deficiencies using motion control. The control of passive AFO elements relies on the activation of springs, valves, or switches in an open-loop manner as the individual walks. This type of AFO has limited robustness and does not adapt to changing walking conditions.
Semi-active devices can store energy, such as in a spring, and provide braking assistance but do not add energy into the system to aid propulsion. Active devices provide assistance in propulsive movements necessary for normal gait. Particular active devices that provide assistance in propulsive phases of gait have been developed for clinical or laboratory settings and are tethered to power sources. Such devices cannot be used outside the clinic or laboratory. Typical active and semi-active devices use large electromechanical actuators that are cumbersome and unattractive.
Compactness and weight are critical to daily use, and current commercial orthoses are all passive as a result. These include passive articulated or non-articulated orthoses, which are made from materials including metal and leather systems, thermoplastics, composites, and hybrid systems. Traditional metal and leather systems have articulated hinge joints with various types of mechanical steps used to limit motion. Some orthoses include springs to resist or assist movement. Common passive devices inhibit motion at undesirable times. Common and more newly developed semi-active devices can also stop or resist motion at undesirable points and only store energy provided by a user, which may not be ideal for treating many gait impairments.